1. Field of the Invention
This invention relates to an air ring for controlling gauge uniformity in the manufacture of blown plastic film.
2. Description of the Related Art
Production of plastic foils by film bubble blow extrusion systems is well known. A lot of publications deal with developments in this technical field, including U.S. Pat. No. 6,702,563 B2, U.S. Pat. No. 5,951,926 and U.S. Pat. No. 5,505,601.
High end blown film extrusion devices are provided with a means for closed loop control of the width of the foil which is extruded. As a consequence, the thickness profile of the blown film bubble is measured over its circumference.
U.S. Pat. No. 5,135,689 shows such a sensor or measuring device. Based on the measurements, the viscosity of the just-extruded plastic is lowered in areas of the circumference of the foil tube in which the foil is too thick or increased in areas in which it is too thin. Then, the thermoplastic foil material is stretched to its final size by the air pressure inside the foil tube. By that, the areas of the foil tube with a low viscosity are further stretched than are the areas with high viscosity so that the width of the foil is balanced and nearly even over the circumference of the foil tube.
The above depicted influence of the viscosity of the extruded plastic material is preferably performed by a local variation of the temperature of the air which is blown against the foil to cool the just-extruded foil. This method is mentioned in U.S. Pat. No. 5,135,689 and U.S. Pat. No. 5,951,929. Another possibility is to locally vary the volume of the air blown against the foil. A third possibility is to locally vary the temperature of the molt in the extruder die head as shown in U.S. Pat. No. 4,339,403. The local variation of the temperature of the cooling air is described in U.S. Pat. No. 5,288,219. Canadian Patent No. 2,438,493 describes a beneficial system for producing tubular film which uses two of these three possibilities to locally vary the viscosity of the just-extruded foil. The above mentioned U.S. Pat. No. 5,288,219 locally varies the temperature of the cooling air with a cooling air ring. This air ring surrounds the just blown gauge.
The air ring typically comprises a generally annular plenum, an annular orifice for discharging cooling air against an extruded plastic tube passing through the air ring, and a flow path for cooling air between the plenum and the annular orifice.
The air flow path is provided with a plurality of cartridge heaters which extend in the air flow path and which heat the cooling air. Each cartridge heater can be individually supplied with current so that each cartridge heater can have a different temperature which results in a different temperature of the cooling air which passes by different cartridge heaters. By this, the temperature of the cooling air is locally varied around the circumference of the gauge. It is a lasting and constant desire of engineers to further improve the above described closed loop control of the film thickness. Hence, a need exists for an improved structure for increasing control over the gauge uniformity of blown plastic film in a film extrusion device.